1. Field of the Invention
The present invention relates to a high strength hot rolled steel sheet having excellent flash butt weldability, fatigue characteristic and formability, which is useful for automobile parts such as wheel rims, discs or bumpers. It also relates to a process for producing such a steel.
2. Description of the Prior Art
As a measure to reduce the weight of automobile bodies for saving costs for fuels, there has been an attempt to modify the material by employment of a high strength steel material, coupled with an attempt to reduce the size of automobile bodies. A highly effective way for the saving of fuel costs is to reduce the weight of automobile wheels, and extensive studies are being made on the use of a high strength hot rolled steel sheet for the wheel rims and discs. For instance, in the United States, a composite structural type hot rolled steel sheet composed of ferrite plus martensite (i.e. dual phase steel sheet) is regarded to be most suitable, and trial tests thereon are being carried out. However, there are certain problems with respect to the characteristics of the material, and there has not yet been a practical application thereof.
Namely, the above mentioned dual phase steel sheet has difficulties such that when it is applied to a wheel disc, fractures are likely to occur at the bore-expanded portion at the time of forming, and it is inferior in the formability, especially in the stretch-flanging property. Further, its fatigue characteristic is not good enough. In its application to a wheel rim, there is a problem such that the portion affected by the welding heat during flash butt welding tends to soften and during the subsequent forming step, necking or fracture is likely to start from this portion.
As references of interest, there are U.S. Pat. No. 3,902,927 to Pernstal and U.S. Pat. No. 3,930,907 to Baily.